In HVDC power transmission networks, alternating current (AC) electrical power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion means that it is not necessary to compensate for AC capacitive load effects that are otherwise imposed by the transmission line or cable. This in turn reduces the cost per kilometer of the lines and/or cables, and thus conversion of AC power to DC power becomes cost-effective when power needs to be transmitted over a long distance.
The conversion of AC power to DC power is also commonly utilized in power transmission networks in circumstances where it is necessary to interconnect two AC networks operating at different frequencies.
Converters are required at each interface between AC and DC networks to effect the required conversion between AC power and DC power, and one such form of converter is a voltage source converter (VSC).
A DC power grid is also needed to support the emergence of HVDC power transmission. The DC power grid includes DC transmission and distribution networks, which may operate at different voltage levels. In such circumstances a voltage source converter may also be used to interconnect the two DC networks.
During operation of HVDC power transmission networks, voltage source converters may however be vulnerable to DC side faults that present a short circuit with low impedance across the DC power transmission lines or cables. Such faults can occur due to damage or breakdown of insulation, movement of conductors or other accidental bridging between conductors by a foreign object.
The presence of low impedance across the DC power transmission lines or cables is detrimental to a voltage source converter because it can cause current flowing in the voltage source converter to increase to a fault current level many times above its original value. In circumstances where the voltage source converter was only designed to tolerate levels of current below the level of the fault current, such a high fault current damages components of the voltage source converter.
Conventionally, in order to reduce the risk posed by a short circuit to a device, one or more switches would be opened to switch the device out of circuit. However the switching elements of voltage source converters, such as the voltage source converter 10 shown in FIG. 1, typically include anti-parallel diodes 14 that remain in conduction when the switching elements 12 are opened. Consequently, even when the switching elements 12 are opened, the diodes 14 allow the fault current 16 arising from a short circuit 18 in a DC network 20 connected to the voltage source converter 10 to flow continuously through the converter 10.
Another option for reducing the risk posed to a voltage source converter by a short circuit is to design the voltage source converter to tolerate the resultant fault current so that there is sufficient time to detect the fault and extinguish the current by opening a circuit breaker on the other, AC side of the voltage source converter.
However the fault current arising from a short circuit in a DC network connected to the voltage source converter is typically many times greater than the rated value of the converter. In order to increase the tolerance of the voltage source converter, either the size and capacity of conducting converter diodes must be increased, several converter diodes must be connected in parallel or a fast-acting bypass device must be incorporated that is capable of carrying the high fault current. In any case, whichever option is pursued, additional inductive components are almost certainly required to limit the high fault current and the increase in components leads to an increase in converter size and weight. This in turn leads to an increase in the size and area of the associated HVDC converter station.
In addition, opening a circuit breaker on the opposite, non-fault side of the voltage source converter is disadvantageous because it disconnects the other network from the HVDC power transmission network. Consequently after the fault is repaired, the converter station must go through a start-up sequence and a series of checks before the other network can be reconnected to the HVDC power transmission network. This leads to a prolonged interruption of power flow and therefore non-availability of the power transmission scheme to those dependent on the scheme for electrical power supply.